Temperature compensated crystal oscillator

ABSTRACT

The present invention relates to an integrated circuit for a temperature compensated crystal oscillator having an external crystal. The integrated circuit comprises a temperature compensation having one fixed or at least two selectable 3 rd  and/or 4 th  and/or 5 th  and/or higher order temperature compensation functions for at least one specific type of external crystal. The temperature compensation can be calibrated at one temperature, in other words without use of temperature variation, by means of an external voltage or current source overdriving a respective temperature-dependent voltage or current supplied from an internal temperature sensor to the temperature compensation.

FIELD OF THE INVENTION

The invention relates to temperature compensated crystal oscillators.

BACKGROUND OF THE INVENTION

Crystal oscillators (XO) are widely used in electronics as highly stableand accurate frequency sources. In a voltage controlled crystaloscillator (VCXO) the nominal oscillation frequency is adjustable inresponse to a voltage control input. The frequency accuracy of thecrystal oscillators is affected by many variables, some of which aretemperature, aging, drive level, retrace and vibration. As illustratedin FIG. 1, normal quartz crystal has quite large temperature variations,which depend on the cutting angle of the crystal.

At many applications the requirements for the maximum temperaturevariations are much tighter than this variation, and therefore varioustemperature compensation methods have been developed. One way to achievefrequency stability is to thermally isolate the crystal and oscillatorcircuitry from ambient temperatures excursions. In an oven controlledcrystal oscillator (OCXO) the crystal and other temperature sensitivecomponents are in a stable oven (a small usually metallic, insulatedenclosure) provided with a heating element and a control mechanism toregulate the amount of heat applied thereby maintaining a constantelevated temperature. However, the OCXOs have disadvantages, such as thespace required for the oven.

A temperature compensated crystal oscillator (TCXO) and a voltagecontrolled temperature compensated crystal oscillator (VCTCXO) typicallycontain a temperature compensation circuit to sense the ambienttemperature and control the crystal frequency in order to prevent thefrequency drift over the temperature range.

As illustrated in FIG. 1, normal quartz crystal has quite largetemperature variations, which depend on the cutting angle of thecrystal. The problem is that present manufacturing methods of the TCXOmodules require temperature variations for accurate temperaturecompensation result. Temperature variations requires expensive ovens,which increases the manufacturing time and costs of the TCXO modules. Inthe TCXO modules the temperature compensation function is implementedwith an integrated circuit or discrete components, which typically havequite large manufacturing tolerances. This means that the TCXO modulehas to be measured at several temperatures to get the correct settingsfor the perfect compensation result.

DISCLOSURE OF THE INVENTION

An object of the present invention is new temperature compensation andmanufacturing method for crystal oscillators.

The invention is characterized by what is stated in the independentclaims. The preferred embodiments of the invention are disclosed in thedependent claims.

An integrated circuit for a temperature compensated crystal oscillatorhaving an external crystal. The integrated circuit comprises temperaturecompensation having one fixed or at least two selectable 3^(rd) and/or4^(th) and/or 5^(th) and/or higher order temperature compensationfunctions for at least one specific type of external crystal, and meansfor calibrating the temperature compensation at one temperature withoutuse of temperature variation. The specific type of external crystalmeans, for example, that if certain type of crystals is used for examplewith enough small manufacturing tolerances or crystals which has beenmeasured at temperatures, the required temperature compensation functionis known. If this compensation function is done with components orintegrated circuit which have a fixed or selectable 3^(rd) and/or 4^(th)and/or 5^(th) order compensation gain matching to the crystal functionand no temperature offset error, a temperature compensated crystaloscillator module does not require any other calculations or temperaturevariations. This means that a temperature compensated crystal oscillatormodule can be manufactured without the use of ovens, which reduces themanufacturing time and costs.

With integrated circuits the manufacturing variations usually has to becalibrated or otherwise the production yield is too low. Thiscalibration can be done at either component testing or at a temperaturecompensated crystal oscillator module testing. The temperaturemeasurement of the temperature compensation function may be done withsome type of temperature sensor, which can be, but is not limited to, aresistor or PN-junction voltage. So the output of such sensor can be avoltage or a current which can be forced externally at the calibrationto correspond to the test temperature. As a result, the temperatureoffset and/or 1^(st) and/or 3^(rd) and/or 4^(th) and/or 5^(th) ordererrors of the temperature compensation block can be measured andcalibrated at one temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail bymeans of preferred embodiments with reference to the attached drawings,in which

FIG. 1 shows quartz crystal temperature behaviour with different cuttingangles;

FIG. 2 shows the orientation of the X, Y, and Z-axis in crystal bar;

FIG. 3 shows simply-rotated cut and doubly-rotated cut in a rectangularcoordinate system; and

FIG. 4 shows an example of an oscillator according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Because the physical properties of crystals vary with orientation, areference orientation and measurement system is necessary. Severalsystems of axial orientation exist; however, the rectangular ororthogonal coordinate system is most commonly used to describe crystalpiezoelectric and mechanical properties. In the rectangular coordinatesystems, the Z-axis is parallel to the m prism faces. FIG. 2 shows theorientation of the X, Y, and Z-axis. A plate of quartz cut with itsmajor surface perpendicular to the X-axis is called an X-cut plate.Rotating the cut 90 degrees about the Z-axis gives a Y-cut plate withthe Y-axis now perpendicular to the major surface. Because a quartzcrystal has six prism faces, three choices exist for the X and Y-axis.The selection is arbitrary; each behaves identically. The piezoelectriceffect determines the sense, positive or negative, of the X and Y-axis.An x-cut plate under stress develops a positive charge on one side ofthe plate and an equal negative charge on the other side. Following theIEEE standard, a positive strain develops a positive charge on thepositive X-face. In this convention, positive strain is defined asextension resulting from tension. Compression on the other hand, createsa negative strain, so the positive direction face of an X-cut plateunder compression is negatively charged. Force applied in the wrongdirection generates no charge. A Y-cut plate does not respond to Y-axiscompression or tension but does respond to shear stress applied to itsedge. Shear stress in a Y-cut plate translates to tension in the Xdirection.

Orientation of the cut with respect to the crystal axis determines notonly value of the physical properties of the crystal, but also theirtemperature coefficient. Changing the crystal orientation by 90 degreeschanges the frequency-temperature coefficient from negative to positive.Partway between the X-cut and Y-cut orientations, the coefficient goesthrough zero. Similarly, a partial rotation of a Y-cut plate about theX-axis yields two points of a zero frequency-temperature coefficient.These orientations are the AT and BT-cuts with rotation angles ofapproximately +35° and 49° respectively.

The lines sloping left from the x-axis mark the saw cut position for ATplates, the line sloping to the right indicates the BT-cut. Referring toFIG. 3, singly rotated cuts are framed by aligning the saw blade withthe crystal X-Z plane (Y-cut) then rotating the blade about the X-axisto the desired angle θ. Preceding the X-axis rotation θ with a rotationφ about the Z-axis as illustrated below in FIG. 3 produces a doublyrotated cut. The majority of crystals manufactured are AT-cuts, however,doubly rotated cuts, especially SC-cuts, are becoming increasinglypopular in moderate and high precision applications.

FIG. 1 gives the frequency-temperature curves for AT-cut fundamentalcrystals. Each curve represents a cut angle Δθ in relation to the basicAT cut (e.g. θ=35° 20′, φ=0) and follows a cubic equation. Temperaturecoefficient, measured as frequency deviation in parts per million (ppm)per degree centigrade corresponds to the slope of the curve. At twopoints the temperature coefficient is zero. These points are the upperand lower turn points and they fall symmetrically about a point in the+20 to +30° C. range. Thus, when one turn point is located by selectingthe crystal cut angle, the position of the other is also fixed. It isnot possible to set the turn points independently. Since moving the turnpoints together reduces the slope between them, frequency stability isoptimized for a given temperature range by selecting the crystal cutangle that places the turn points towards the ends of, or just beyondthe expected temperature extremes. With the turn point axis of symmetryin the range of +20 to +30° C., excellent frequency stability can beachieved without compensation for modest temperature excursions aboutroom temperature. Curve 0 in FIG. 1 is nearly flat throughout thisrange.

Quartz crystal temperature behavior can be modeled as follows:dfT(T,a1,a3,a4,a5,Tinf):=a1·(T−Tinf)+a3·(T−Tinf)³ +a4·(T−Tinf)⁴a5·(T−Tinf)⁵  (1)

Where T=Temperature; a1, a3, a4, a5=coefficients for 1st, 3rd, 4th and5th order temperature errors; Tinf=Temperature offset error, zero pointof temperature curve derivative.

FIG. 4 illustrates an example of a VCTCXO which implements theprinciples of the present invention. The oscillator 40 may beimplemented as an integrated circuit well suited for variousapplications, such as the mobile phones and other telecommunicationssystems. Only a crystal X1 may be required as an additional externalcomponent. A resonant circuit includes differential amplifiers A1 andA2, the crystal X1, a varactor diode VD1, and the voltage controlledcapacitances C1 and C2. A voltage control input VC receives a controlvoltage which is feed through a temperature compensation 401 to adjustthe reverse voltage across the varactor VD1 and thereby the capacitanceof VD1 and the resonant frequency of the resonant circuit. Thetemperature compensation 401 has a fixed or selectable 3^(rd) and/or4^(th) and/or 5^(th) order compensation gain (i.e. coefficients a1, a2and a3 in the equation 1 above) matching to frequency-temperaturefunction of the crystal X1. The 3^(rd) and/or 4^(th) and/or 5^(th) ordercompensation functions are depicted with blocks 402, 403, and 404 inFIG. 4. The temperature compensation 401 has no temperature offset errorcompensation, i.e. the linear gain a1 in the equation 1. Thecompensation signals produced by the blocks 402, 403, and 404 are summedwith the control voltage VC so as to achieve the actual, compensatedcontrol voltage. The ambient temperature measurement of the compensationblock 401 is done with a temperature sensor 405. The temperature sensor405 can be, but is not limited to, a resistor or PN-junction voltage, inwhich case the output of the sensor 405 to the either voltage orcurrent. The oscillator circuit 40 is calibrated, programmed andcontrolled through a serial bus of a data block 402. The serial buscomprises a programming input PV, a serial bus clock input CLK and aserial bus data input DA.

With integrated circuits the manufacturing variations usually has to becalibrated or otherwise the production yield is too low. Thiscalibration can be done at either component testing or at the TCXOmodule testing. The circuit 40 has internal temperature sensor 405,whose output TSO1 is temperature dependent voltage or current. Atcalibration situation the temperature changes are modeled with anexternal voltage or current source that is connected to the output pinTSO1 of the temperature sensor. Thereby the temperature sensor outputTSO1 can be forced to some known voltages with an externalvoltage/current source. The integrated circuit 40 can be calibrated bymeasuring the compensation curve of the integrated circuit (e.g. the sumof all compensation terms) voltage or output frequency with some knownTSO1 voltages. The required settings of the integrated circuit 40 can becalculated from these measurements so that circuit matches to somecertain crystal parameters.

For example: The temperature offset of the integrated circuit 40 may becalibrated by measuring the compensation curve of the integrated circuit40 at five TSOI1 points, such as default settings of an internaldigital-analog converter (DAC). The internal DAC provides a lineartuning range for the temperature calibration. Thus, when the temperatureoffset error from the desired nominal value or default value ismeasured, the correct DAC setting can be calculated from the followingequation:DA−setting=(Default value)−(error/DAC step)

The compensation gain of the integrated circuit 40 can be calibrated bythe same method. For example, the calibration can be performed by meansof measuring the output frequency at two known TSO1 points. Thefrequency difference between these two points is then subtracted fromthe desired value or default value that depends on the crystal. Then thecorrect DAC setting can be calculated from the above equation. The DACsettings or other calculated calibration values are programmed in thedata block 402 through the serial bus. The calibration of the TCXOmodule does not require any other calculations or temperaturevariations. Thus, the temperature offset and 1^(st) and/or 3^(rd) and/or4^(th) and/or 5^(th) order errors of the temperature compensation blockcan be measured and calibrated at one temperature. This means that TCXOmodule can be manufactured without the use of ovens, which reduces themanufacturing time and costs.

Crystals for which the required temperature compensation function isknown are used as the crystal X1, such as certain type of crystals withenough small manufacturing tolerances, or crystals which has beenmeasured at temperatures. Advantageously, AT cut crystals with a cutangle of approximately 0 degrees, or within approximately ±1 degrees canbe used. As can be seen in FIG. 1, the 0 degrees curve is nearly flatthroughout in the range of +20 to +30° C., excellent frequency stabilitycan be achieved without compensation for modest temperature excursionsabout room temperature. Thus, it is possible to perform accuratecalibration in a room temperature, or more generally, less accurateambient temperature is required for the calibration.

In case of the crystal being a AT cut crystal with a cut angle ofapproximately 0 degrees, the temperature compensation 401 may have afixed, preprogrammed compensation function, e.g. fixed 3^(rd) and/or4^(th) and/or 5^(th) order compensation gain blocks matching to thepredetermined, known frequency-temperature function of the specific ATcut crystal with a cut angle of approximately 0 degrees. Thus, only thetemperature offset and 1^(st) and/or 3^(rd) and/or 4^(th) and/or 5^(th)order errors of the temperature compensation block may be measured andcalibrated at one temperature.

In case of selectable compensation functions, the temperaturecompensation 401 may have two or more fixed, preprogrammed compensationfunctions matching to the predetermined, known frequency-temperaturefunctions of two or more different types of crystals. For example, thetemperature compensation 401 may have a first fixed, preprogrammedcompensation function for an AT cut crystal with a cut angle ofapproximately 0 degrees, a second fixed, preprogrammed compensationfunction for an AT cut crystal with a cut angle of approximately +1degrees, and a third fixed, preprogrammed compensation function for anAT cut crystal with a cut angle of approximately +1 degrees. Themanufacturer of the TCXO or VCTCXO can choose one of these crystal typesfor the oscillator and select the respective compensation function inthe temperature compensation 401 through the serial bus during thecalibration. The manufacturer has more freedom to select the crystal,while the calibration is still simple and can be performed at onetemperature.

It will be obvious to a person skilled in the art that, as thetechnology advances, the inventive concept can be implemented in variousways. The invention and its embodiments are not limited to the examplesdescribed above but may vary within the scope of the claims.

1. An integrated circuit for a temperature compensated crystaloscillator having an external crystal, the circuit comprising aninternal temperature sensor, a temperature compensator having one fixedor at least two selectable 3^(rd) and/or 4^(th) and/or 5^(th) and/orhigher order temperature compensation functions for at least onespecific type of external crystal, and calibrator circuitry for thetemperature compensator, said calibrator circuitry being configured tobe connected an external voltage or current source for calibrationprocedure of the temperature compensator, said external voltage orcurrent source overdriving a respective temperature-dependent voltage orcurrent supplied from said internal temperature sensor to thetemperature compensator, whereby enabling the calibration procedure tobe performed at one temperature without use of temperature variation. 2.A temperature compensated crystal oscillator as claimed in claim 1,wherein said oscillator is a voltage controlled temperature compensatedcrystal oscillator.
 3. An integrated circuit as claimed in claim 1,wherein said calibrator circuitry comprise means for calibrating atemperature offset error at one temperature without use of temperaturevariation.
 4. An integrated circuit as claimed in claim 1, wherein saidcalibrator circuitry comprise means for calibrating 1st and/or 3rdand/or 4th and/or 5th and/or higher order temperature errors at onetemperature without use of temperature variation.
 5. An integratedcircuit as claimed in claim 1, wherein said calibrator circuitry isadapted to be used for calibration at component or module testing at onetemperature.
 6. An integrated circuit as claimed in claim 1, wherein atemperature offset error of said integrated circuit is pre-calibrated atcomponent or module testing at one temperature using said calibrationmeans.
 7. An integrated circuit as claimed in claim 1, wherein 1stand/or 3rd and/or 4th and/or 5th and/or higher order temperature errorsof said compensator are pre-calibrated at component or module testing atone temperature by using said calibration means.
 8. An integratedcircuit as claimed in claim 1, comprising means for programming andstoring the calibration values obtained during the calibration.
 9. Anintegrated circuit as claimed in claim 1, wherein temperaturecompensator comprise a fixed 3^(rd) and/or 4^(th) and/or 5^(th) and/orhigher order temperature compensation function for an AT-cut crystalwith a cutting angle of approximately 0 degrees or within range ofapproximately ±1 degrees.
 10. An integrated circuit as claimed in claim1, wherein temperature compensator comprise a first selectable 3^(rd)and/or 4^(th) and/or 5^(th) and/or higher order temperature compensationfunction for an AT-cut crystal with a cutting angle of approximately 0degrees, a second selectable compensation function for an AT cut crystalwith a second cut angle within range of approximately ±1 degrees, and athird selectable compensation function for an AT cut crystal with athird cut angle within range of approximately ±1 degrees.